Ligand-exchange enantioresolution of dihydroisoxazole amino acid derivatives acting as glutamatergic modulators

Four dihydroisoxazole prolines (1-4) and four dihydroisoxazole cyclopentane (5-8) derivatives, evaluated as glutamatergic modulators, were submitted to chiral ligand-exchange chromatography (CLEC) analysis. A chiral stationary phase (CSP) obtained through the coating of C-18 chains with S-trityl-(R)-cysteine [(R)-STC] failed the enantioseparation of compounds 3, 4, 7 and 8. On the other hand, a chiral mobile phase (CMP) system from O-benzyl-(S)-serine [(S)-OBS] coupled with Cu(II) nitrate as the metal source into the eluent, produced an excellent enantioseparation of all compounds. Worthy to be mentioned is the results achieved for compound 3: α = 7.26 and RS = 25.75. The established CMP-CLEC method was fully validated with the enantiomeric couple 2 as model compound. In all the cases, very good precision (RSD% values ranging from 1.77 to 4.09% in the long-period) and accuracy (Recovery% values ranging from 97.58 to 99.92% in the long-period) values along with appreciably low LOD (25.61 μg mL-1, and 24.79 μg mL-1, for the first and second eluted enantiomer, respectively) and LOQ (77.62 μg mL-1, and 75.08 μg mL-1, for the first and second eluted enantiomer, respectively) values turned out. The statistical relevance of the developed CMP-CLEC method allowed us to successfully undertake the semi-preparative scale enantioisolation of compounds 7 and 8. As a result of a quantitative structure-property relationship (QSPR) study, the van der Waals energy was found to be the most suitable descriptor for explaining the retention behaviour with the (S)-OBS-based system, outlining the prominent role of hydrophobic and dispersive interactions in this CMP-CLEC environment.